DETAILED DESCRIPTION
The GT4122 is a broadcast quality monolithic inte-
grated circuit specifically designed to linearly mix two
video signals under the control of a third signal.
When they are exactly 0.5V and when VC = VREF, the gain of
each signal channel of the mixer is 0.5 (50%).
By connecting the ends of an external potentiometer
(CONTROL OFFSET) between the offset pins COS1 and
COS2, the voltage sources can be altered differentially. If a
second potentiometer (50% GAIN) is connected between the
wiper of the CONTROL OFFSET potentiometer and the supply
voltage, the voltage sources can be varied in a common
mode fashion. In this way not only can the control range of
the mixer be varied but also the point at which 50% of each
input signal appears at the output.
Referring to the block diagram, the input signals are applied
to conventional differential amplifiers (AMP1 and AMP2).
Each amplifier has provisions for individually adjusting the
DC offset (OFFSET).
Following each input amplifier, the signals are applied to
linear multiplier circuits (XA and XB) whose outputs are the
product of the incoming signals and controlling voltages (VCA)
or (VCB). The controlling voltage VCA is the sum of a nominal
0.5V source (VNOM) and a variable source VK while VCB is made
up of the sum of the nominal voltage VNOM and -VK.
The outputs from the multiplier circuits (XA and XB) are then
applied to a summing circuit (Σ1) whose output feeds a
wideband amplifier (AMP4) and presents the mixed signals to
the outside world.
VK and -VK are themselves proportional to the difference
between an externally applied reference voltage (VREF) and
an externally applied CONTROL voltage (VC). The voltages VK
and -VK are produced by a differential amplifier (AMP3) whose
gain is AK. This gain can be altered by two external resistors,
Although there are two separate differential inputs, the usual
operational amplifier gain-setting methods can be applied to
determine the closed loop gain of the mixer. Usually the
mixer will be configured for unity gain by connecting both
inverting inputs (-IN A , -IN B) to the common output (OUT). In
this case, the general transfer function is:
REXT and RSPAN according to the following formula:
0.85 • REXT
AK ≈ —————
[1kΩ < REXT < 3kΩ]
RSPAN
VO = VA •[VNOM + AK•(VC - VREF)] + VB•[VNOM - AK•(VC
REF)]
-
V
Note that REXT is connected between the REXT pin and ground
and RSPAN is connected between the pins S1 and S2.
(Unity gain configuration)
Each of the voltages (+VK and -VK) is applied to summing
circuits (Σ2 and Σ3) whose second inputs are DC voltage
sources that can also be slightly varied. The nominal value of
these voltage sources is 0.5 volts.
Where VA and VB are the input analog signals applied to +IN A
and +IN B respectively, and VC is the CONTROL voltage.
Note that VNOM ranges between 0.45V < VNOM < 0.55.
+
-
+
IN A
IN A
AMP 1
XA
-
COMP
V
=0.5 + V
CA
K
+
A
OS1
OS2
IN B
OUT
AMP 4
BIAS
Σ1
A
+
+
-
+
R
EXT
AMP 2
XB
-
IN B
OS1
OS2
+
+
-
V
K
+
+
V
B
Σ2
Σ3
AMP 3
V
=0.5 - V
CB
K
-
V
REF
B
V
K
A
K
+
V
C
-
V
S
0.5V
V
NOM
NOM
+
-
+
-
S1
+V
S
S 2
GND
C
OS2
C
OS1
Device Function: VOUT = VINA • [VNOM + AK (VC - VREF)] + VINB • [VNOM - AK (VC - VREF)]
FUNCTIONAL BLOCK DIAGRAM
520 - 25 - 4
3
GENNUM [ GENNUM CORPORATION ]
